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When I was going through the chapter on pnicotgens, I got to know that ammonia acts as a Lewis base and it dissociates in water giving out hydroxide ion and the further hydroxide of ammonia, which we refer to as ammonium hydroxide, precipitates metal salts in a form of metal hydroxides.

I want to know the reason why this thing happens. According to me, ammonium hydroxide should form a complex with the metal being a base after donating a pair of electrons. Correct me if I am wrong. A reaction is given below as an example of a reaction between ammonium hydroxide and zinc sulfate:

$$\ce{ZnSO4 + 2 (NH3 * H2O) → Zn(OH)2 + (NH4)2SO4}$$

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    $\begingroup$ Unfortunately, solubility is nowhere near that simple. $\endgroup$
    – Zhe
    Feb 13, 2019 at 14:25
  • $\begingroup$ Zinc forms ammine complex $\ce{[Zn(NH3)4]^2+}$ all right in liquid ammonia and when exposed to ammonia gas. It just so happens that in aqueous solution zincate $\ce{[Zn(OH)4]^2−}$ formation is favored. $\endgroup$
    – andselisk ♦
    Feb 13, 2019 at 14:28
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    $\begingroup$ Not so much $\ce{Zn(OH)_4^{2-}}$ as precipitated $\ce{Zn(OH)_2(s)}$. Getting the hydroxy-complex anion in large amounts requires a stronger basic solution than that formed by ammonia in water. $\endgroup$
    – Oscar Lanzi
    Feb 14, 2019 at 14:55
  • $\begingroup$ Not sure what you are looking for. Ammonia is a ligand that can help solubilize metal ions. Exactly when that happens or doesn't happen isn't that easy to predict from first principles. In this case, you have several equilibria in play which further complicates the situation. $\endgroup$
    – Zhe
    Feb 15, 2019 at 13:03
  • $\begingroup$ I just want to know that why Ammonium hydroxide precipitate out the metal salt what's the reason behind that because in some metals Ammonia form complexes whereas in others it precipitate them out why this difference arises is it due to the configuration of different metals I mean is it due to the different nature of metals or something else thanks in advance. $\endgroup$
    – David Wax
    Feb 15, 2019 at 13:16

2 Answers 2

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It's simply because of the concentration. In aqueous solution, the formation of a precipitate is favored over the formation of a complex ion, and it can be shown using solubility equilibria and $K_\mathrm{f}$ (but it won't be pretty).

This is just like the QA test for copper(II). After a small amount of aqueous ammonia is added, $\ce{Cu(OH)2}$ is observed. Add more, and you get the deep blue $\ce{[Cu(NH3)4]^2+}$ complex.

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Zinc has amphoteric properties like aluminum. It can dissolve in bases as well as acids. In bases it forms zincate anions. The "why" part of science often has no answer because the reason is that nobody fully understands such a complex phenomenon. It is relatively easier to talk about the observational part.

An aqueous solution of ammonium is very basic, which means that there are free hydroxide ions there. Writing NH3.H2O is deceptive because many general chemistry texts emphasize that NH4OH does not exist and they advocate NH3.H2O. One way to rationalize the observation of the Ksp values for zinc hydroxide i.e. if we have free zinc ions and free hydroxide ions, a solid Zn(OH)2 is favored at that particular pH.

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